Blender image output is greyish and not white
Why is this happening?

The image's colorspace, and the color space that blender is using to display them are not the same.

Images displayed in a different color space will always look wrong

The values for what you call "white" are different depending on the color transform you use in the color management section.

If you are dealing with video files only, using sRGB values, and not rendering anything that uses a wide dynamic range, then you don't need "filmic", which is the "default" option on blender.

Set the View transfrorm from Filmic to sRGB (or "standard") in the color management section. To understand the difference read the following links:

White background with filmic blender

and

Render with a wider dynamic range in cycles to produce photorealistic looking images